The present invention generally relates to a printing apparatus of the k type which is used in various types of printers, for copying machines, in telefacsimile machines etc., and which operates with a dry print powder which is, in electrical way, applied to the object to be printed, for instance the paper, and which is thereafter fixed to the paper, generally by a heat treatment.
The invention is more particularly directed to a printing apparatus of said type which named a xe2x80x9ctoner jetxe2x80x9d printing apparatus, and in which a dry print powder, generally named xe2x80x9ctonerxe2x80x9d, is, by a direct method, transferred from a rotating toner feeder roll, through bores of a fixed matrix in the form of a flexible printing circuit and down onto the object to be printed, for instance the paper which is conveyed over a support roll and in which the print powder which has been applied to the paper is finally fixed to the paper by a heat treatment.
The basis of said process is that two electrical fields are created for transferring toner from the toner feeder roll to the paper, namely a first electrical field between the toner feeder roll and the toner matrix, which electrical field is brought to invert polarity, and a second, preferably constantly downwards directed positive electrical field between the matrix and the support roll above which the paper is transferred.
The toner matrix is formed with a large number of very small through bores having a diameter of for instance 100-300 xcexcm, and round each individual such bore an electrically conducting ring of a suitable metal, like copper, in the following referred to as xe2x80x9ccopper ringxe2x80x9d. Each copper ring can be charged with a positive potential, for instance +300V, which is higher than the potential of the toner feeder roll, which potential can for instance be between +5V and +100V, preferably about +50V, but which is less than the potential of the support roll for the paper, which potential can for instance be +1500V. When the electrically conducting ring is charged with a voltage said ring makes the belonging matrix bore become xe2x80x9copenedxe2x80x9d for letting toner down. If the matrix bore ring is, on the contrary, charged with a potential which is substantially lower than the potential of the toner feeder roll, for instance in that said ring is connected to ground, the belonging matrix bore becomes xe2x80x9cclosedxe2x80x9d thereby preventing a letting down of toner.
The function is as follows:
the toner powder gets a negative potential in that said toner particles rub against each other;
the toner powder is supplied to the toner feeder roll, which is positively charged by a predetermined potential, often a potential which can be controlled between +0V and +100V, and the toner powder is distributed in an even, sufficiently thick layer over the toner feeder roll using a doctor blade;
each bore of the matrix which corresponds to a desired toner dot is opened in that the matrix bore ring is charged with a positive potential which is higher than the potential of the toner feeder roll, for instance +300V; bores corresponding to non toner carrying portions remain connected to the ground, whereby said bores are to be considered xe2x80x9cclosedxe2x80x9d, thereby making it impossible to let toner through; the combination of opened matrix bores forms the image to be reproduced;
depending on the difference in potential, for instance +50V to +300V=250V between the toner feeder roll and the toner matrix negatively charged toner particles are sucked down from the toner feeder roll to the matrix, and depending on the difference in potential between the toner matrix and the support roll mounted underneath same, for instance +300V to +1500V=+1200V the toner particles are moved on from the matrix and deposit on the paper above the support roll;
the paper with toner deposited thereon is finally moved through a heat treatment apparatus in which the toner is fixed to the paper.
There is an almost linear relationship between the density of the current field and the traction force that said field exerts on the toner particles. The field has its greatest density just above the copper rings, and the density decreases from the ring edges towards the center of the bore. By reducing the potential of the toner feeder roll, which leads to an increasing difference in potential between the toner feeder roll and the matrix, it is possible to increase the amount of toner which is let down. An increase of the potential of the toner feeder roil leads a corresponding reduction of the amount of toner which is let down.
By connecting the copper ring of the matrix to the ground the direction of potential between the toner feeder roll is reversed from having been +250V in the direction downwards to be +50V in the direction upwards, and this makes negatively charged toner particles stick to the toner feeder roll, or be sucked back thereto, respectively.
In a certain embodiment of the printing apparatus the distance between the toner feeder roll and the matrix was about 0.1 mm, and the distance between the matrix and the support roll was about 0.6 mm. At normal printing the toner feeder roll has a voltage of +50V, and this gives a difference in potential to the matrix, which can have a voltage of +300V, of +250V between the toner feeder roll and the matrix. over the above mentioned distance of 0.1 mm this gives a field strength of 2.5V/xcexcm.
The distance between the toner feeder roll and the support roll is about 0.7 xcexcm, and the difference in potential is +1450V. This gives a field strength of 2 V/xcexcm between the bottom surface of the matrix and the paper. The same electric field is present above the matrix and between the copper rings, and said field acts against the toner on the toner feeder roll, so that toner particles can be released from the toner feeder roll and can fall down on the upper surface of the matrix. As soon as the toner particles reach a copper ring, which is connected to ground (0V), said toner particles jump back to the toner feeder roll, and after having passed the copper ring said particles jump back down to the matrix again.
It also can happen that toner which is present above a conduit to a copper ring when the voltage changes from 0V to +300V can be sucked to the upper surface of the matrix and can be kept thereon, and this can prevent other toner particles from being fed into the matrix bore at the centre of the copper ring.
Toner which jumps up and down between the toner feeder roll and the upper surface of the matrix obstacles the flow of toner past the printing zone, and the jumping toner particles are often unloaded or may even change charge to the non-desired positive charge. Also, a slight amount of the toner particles normally have a xe2x80x9cfalsexe2x80x9d potential, generally 2-4% of the toner particles, and such falsely charged toner particles are often sucked both to the upper surface and to the bottom surface of the matrix.
The present invention is intended to solve the problem that toner particles jump between the toner feeder roll and the matrix, and said problem is solved in that a thin, protective metal layer is applied on the upper surface of the matrix. Said protective layer is formed with bores the diameters of which coincide with the outer diameter of the copper rings. The layer is given the same potential as that of the toner feeder roll, for instance +50V. The protective layer can have a thickness of 20-30 xcexcm, and it is glued onto the upper surface of the matrix. The protective metal layer acts as an electric screen between the toner feeder roll and the matrix with the electric conduits thereof.
It is important that the bores of the protective layer each have a diameter which is at least the same as the outer diameter of the copper rings, since there would otherwise be a risque that the layer might screen off the field between the toner feeder roll and copper rings. In order to prevent that the material between the bores of the protective layer is too narrow the matrix is preferably formed with the copper rings on the top of the matrix base and with the inner diameter of the copper ring the same size as that of the bores of the matrix, whereby the copper rings may be used to a maximum for feeding toner particles from the toner feeder roll, through the matrix and down to the paper. In a matrix having a toner feeder bore with a diameter of about 190 xcexcm the copper rings can have an outer diameter of for instance 250 xcexcm, and in such case the bores of the protective layer can preferably be given a diameter of 250 xcexcm.
If the toner feeder roll and the toner is of magnetic type the protective layer has to be of an unmagnetic material like of stainless steel, beryllium copper, hard nickel, brass, aluminum or another hard, unmagnetic material.
In order to eliminate the risque of flash over between the toner feeder roll and the matrix and between the copper rings and the support roll it is therefore necessary that the matrix bore ring be insulated. This is done in that the entire matrix is covered, for instance by an evaporation process, with an insulating substance which encloses all free surfaces and edges of the matrix, the matrix bores and the protective layer. An available method is the method named the Parylene(copyright) method (Union Carbide) according to which a polymeric insulation material named poly-para-xylene, using a vacuum apparatus, is applied to the matrix in a very well predetermined thickness. The material has an electric decomposition resistance of about 200 V/xcexcm. This means that it is sufficient to use a layer having a thickness of only 2 xcexcm for insulating an electric field of +250V between the toner feeder roll and the copper ring of the matrix.